With a recent advent of large-sized LCDs and a gradual expansion of their usage from personal devices, such as mobile phones, lap-top computers, etc., to home appliances, such as wall mounted flat panel TVs, there is a demand for LCDs with high definition and wide viewing angle. In particular, TFT-driven thin film transistor LCDs (TFT-LCDs) of which each pixel is independently driven are much superior in response speed of liquid crystals, realizing high-definition motion pictures, and thus increasingly used in a wider range of applications.
To be used as an optical switch in the TFT-LCDs, liquid crystals are required to be initially aligned in a defined direction on a layer including innermost TFT of the display cell. For this, a liquid crystal alignment layer is used.
For the liquid crystal alignment to occur, a polymer alignment layer is formed and then subjected to a rubbing process using a rotary roller wound with a rubbing cloth of nylon or rayon fabrics at a high rotation speed to align liquid crystals. However, the rubbing process remains mechanical scratches on the surface of the liquid crystal alignment layer or generates strong static electricity, possibly destroying the TFTs. Further, fine fibers coming from the rubbing cloth may cause defects of the products, bringing about an obstacle to acquiring a high production yield.
In an attempt to overcome the problems with the rubbing process and achieve innovation in the aspect of production yield, there has been contrived a liquid crystal alignment method using a light such as UV radiation (hereinafter, referred to as “photo-alignment”).
Photo-alignment refers to the mechanism using a linearly polarized UV radiation to cause the photoreactive groups of a defined photoreactive polymer to participate in a photoreaction, aligning the main chain of the polymer in a defined direction to form a photo-polymerized liquid crystal alignment layer with aligned liquid crystals.
The representative example of the photo-alignment is photopolymerization-based photo-alignment as disclosed by M. Schadt et al. (Jpn. J. Appl. Phys., Vol 31., 1992, 2155), Dae S. Kang et al. (U.S. Pat. No. 5,464,669), and Yuriy Reznikov (Jpn. J. Appl. Phys. Vol. 34, 1995, L1000). The photo-aligned polymers used in these patent and research papers are mostly polycinnamate polymers, such as poly(vinylcinnamate) (PVCN) or poly(vinyl methoxycinnamate) (PVMC). In the case of photo-alignment of the polymers, the double bond of cinnamate exposed to UV radiation participates in a [2+2] cycloaddition reaction to form cyclobutane, which provides anisotropy to cause liquid crystal molecules aligned in one direction, inducing liquid crystal alignment.
However, the conventional photoreactive polymers for alignment layer have their main chain with poor thermal stability or exhibit poor characteristics in regards to photoreactivity, alignment properties, or alignment rate.
To solve this problem, there has been developed a photoreactive polymer having a long spacer added between the main chain and the photoreactive group to improve in photoreactivity and alignment properties, or a photoreactive cyclic-olefin polymer having a photoreactive group with improved alignment properties or the like. Undesirably, the photoreactive polymer having a long spacer added between the main chain and the photoreactive group has poor thermal stability, and the photoreactive cyclic-olefin polymer has a low solubility to organic solvents and consequently poor workability for forming a photo-alignment layer.
On the other hand, there has recently been a demand for using photo-alignment layers in the fields of applications, such as patterned retarders, patterned cell alignment layers, or the like, that require a change of the anisotropic direction based on the polarization direction to create three-dimensional stereoscopic images. But the conventional photoreactive polymers have no change in the alignment direction already determined by the polarized radiation. And, if any change in the alignment direction is made, it is required to use a greater intensity of radiation polarized in a different direction. Accordingly, there is a demand for developing a novel photoreactive polymer having readiness for change in the alignment direction based on the polarization direction of radiation.